In order to mount electronic components onto a circuit board or the like, well-known is a bonding method using solder which contains lead. In recent years, however, knowledge about environmental problems has been increasing; thus, attention has been paid to lead-free solders or adhesive compositions, which contain, no lead, instead of solder.
However, SnAgCu solder, which is mainly used as a lead-free solder, has a problem that the temperature for connection therewith is as high as 260° C. so that the solder is not usable for some electric components high in thermal sensitivity. As a lead-free solder for lower temperature, Sn—Bi solder, the melting point of which is 138° C., is known. However, the solder has a problem that in an article used in a high-temperature environment, the solder is allowed to stand still at high temperature, whereby the composition of the solder is varied so that the solder turns brittle.
In order to overcome these problems, there is suggested a thermosetting type electroconductive adhesive made into a paste form wherein a filler such as silver powder is dispersed in a thermosetting resin (Japanese Patent Application Laid-Open No. 2002-161259). About the thermosetting type electroconductive adhesive, the thermosetting resin is used as a binder component, thereby making it possible to restrain a fall in the strength at high temperature. However, a mechanism for expressing the electroconductivity is based on contact between the metal particles; therefore, in order to ensure a good electroconductivity, it is necessary to increase the amount of the filled electroconductive particles. As a result, there remains a problem that a fall in the adhesive force is caused by a decrease in the amount of the binder component.
Thus, suggested is an electroconductive adhesive wherein a flux and a metal meltable at low temperature, such as lead-free solder, are filled into a thermosetting resin, thereby making the adhesive property and the electroconductivity compatible with each other (see Japanese Patent No. 3730209, and JP-A No. 2001-170797). The solder-containing electroconductive adhesive is excellent in viewpoints that electroconductivity is ensured by metallic bonds of the solder, that a flux-washing step is eliminated by the flux meltability of the thermosetting resin and the heat resistance thereof, and that adhesive property is ensured. However, a halogen-containing flux has a problem of a load onto the environment or the induction of migration, and other problems. A bifunctional carboxylic acid having no hydroxyl group has a problem that the addition amount for forming metallic bonds turns remarkably large so that the storage stability declines or the adhesive property lowers, or other problems.
An object of the invention is to provide an adhesive composition that keeps storage stability and further gives a cured product wherein metallic bonds are formed in the state that the cured product wets its components (or parts) and is satisfactorily spread between the components, thereby turning excellent in adhesive property, electroconductivity, and reliability for mounting such as TCT resistance or high-temperature standing resistance. Another object of the invention is to provide an electronic-component-mounted substrate or semiconductor device having a good reliability.